Rendering an animated Normal Map using Orthographic Camera in cycles
I have made a test setup that works, at least as far as I can tell:
First, it's important to set the Colour Management display type to sRGB/RAW (or to None) and the gamma value to 1.0. This makes sure that the render output is stored into the output file in a linear manner. It shouldn't affect how the RGB values are shown inside of Blender though, so this should always be correct.

Then a glance at how Blender expects normal maps to behave and how it stores the normals internally helps with setting up the material. Information on this can be found here: Wiki: Bump and Normal Maps
Quote:

Red maps from (0-255) to X (-1.0 - 1.0)
Green maps from (0-255) to Y (-1.0 - 1.0)
Blue maps from (0-255) to Z (0.0 - 1.0)

In other places, the blue channel is also mapped from -1.0 to 1.0. Either way, this means that the red and green values have to be remapped accordingly. Optionally, this applies to the blue channel, too. This is easy to achieve using math nodes (here for blue from -1.0 to 1.0):

Then, using an orthographic camera from the top, the follwing result is achieved:

Here, camera-facing surfaces have RGB values of (0.5|0.5|1.0), as expected. Also, the colours stay between 0 and 1 in all areas, but use the full range. So this looks good to me.